U.S. patent number 5,828,722 [Application Number 08/856,147] was granted by the patent office on 1998-10-27 for x-ray diagnostic apparatus for tomosynthesis having a detector that detects positional relationships.
This patent grant is currently assigned to Sirona Dental Systems GmbH & Co., KG. Invention is credited to Manfred Franetzki, Josef Ploetz.
United States Patent |
5,828,722 |
Ploetz , et al. |
October 27, 1998 |
X-ray diagnostic apparatus for tomosynthesis having a detector that
detects positional relationships
Abstract
An diagnostic apparatus for generating X-ray exposures for
tomosynthesis includes a position detector (2, 4) for registering
the mutual positional relationship between the radiation
transmitter (1) and/or the radiation receiver (3) and/or the object
(7) under examination. The position detector (2, 4) can be arranged
in part or in whole on the radiation transmitter (1), on the
radiation receiver (3) or on the object (7) under examination. An
evaluation device (11) evaluates the signals from the position
detector (2, 4) and generates correction signals if the relative
position between the radiation transmitter (1) and/or the radiation
receiver (3) and/or the object (7) under examination deviates from
the home position. The correction signals are then fed to an
image-generating device (12) for use in the image processing.
Inventors: |
Ploetz; Josef (Bensheim,
DE), Franetzki; Manfred (Bensheim, DE) |
Assignee: |
Sirona Dental Systems GmbH &
Co., KG (Paris, FR)
|
Family
ID: |
7794577 |
Appl.
No.: |
08/856,147 |
Filed: |
May 14, 1997 |
Foreign Application Priority Data
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May 17, 1996 [DE] |
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196 19 925.5 |
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Current U.S.
Class: |
378/38; 378/170;
378/205 |
Current CPC
Class: |
A61B
6/547 (20130101); A61B 6/587 (20130101); A61B
6/14 (20130101); A61B 6/08 (20130101); A61B
2090/363 (20160201) |
Current International
Class: |
A61B
6/08 (20060101); A61B 6/14 (20060101); A61B
19/00 (20060101); A61B 006/03 () |
Field of
Search: |
;378/38,39,21,22,25,24,23,168,170,205,204 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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A-3808009 |
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Sep 1988 |
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DE |
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A-4414689 |
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Nov 1995 |
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DE |
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WO-A-9322893 |
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Nov 1993 |
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WO |
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Primary Examiner: Porta; David P.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas, PLLC
Claims
What is claimed is:
1. An X-ray diagnostic apparatus for generating a tomosynthesis
recording, comprising:
a radiation transmitter that transilluminates an object under
examination from a plurality of different directions;
a radiation receiver that is adjustably mounted relative to said
radiation transmitter and that produces radiation signals for
generating the tomosynthesis recording of the object under
examination;
an image-generating device that receives the radiation signals for
each of the plurality of different directions and computes the
tomosynthesis recording of the object under examination;
at least one position detector that produces position signals in
accordance with a positional relationship between at least any two
of (i) said radiation transmitter, (ii) said radiation receiver,
and (iii) the object under examination, wherein said position
detector is mounted on at least any one of said radiation
transmitter, said radiation receiver, or the object under
examination; and
an evaluation device that (i) evaluates the position signals to
determine a deviation between the positional relationship and a
desired relationship, (ii) generates correction signals in
accordance with the evaluated deviation, and (iii) outputs the
correction signals to said image-generating device.
2. The X-ray diagnostic apparatus as claimed in claim 1, wherein
said image-generating device computes the tomosynthesis recording
of the object under examination on the basis of at least the
radiation signals and the correction signals.
3. The X-ray diagnostic apparatus as claimed in claim 1,
wherein:
said position detector is implemented as at least any one of a
magnetic, electromagnetic, optical, acoustic or mechanical
device;
said position detector comprises a transmitter and a receiver;
said transmitter of said position detector is mounted on at least
one of said radiation receiver or the object under examination;
and
said receiver of said position detector is arranged on said
radiation transmitter.
4. The X-ray diagnostic apparatus as claimed in claim 1,
wherein:
said position detector is implemented as at least any one of a
magnetic, electromagnetic, optical, acoustic or mechanical
device;
said position detector comprises a transmitter and a receiver;
said transmitter of said position detector is arranged on said
radiation transmitter; and
said receiver of said position detector is mounted on at least one
of said radiation receiver or the object under examination.
5. The X-ray diagnostic apparatus as claimed in claim 1, further
comprising:
a holding device for holding the object to be examined;
wherein:
said position detector is implemented as at least any one of a
magnetic, electromagnetic, optical, acoustic or mechanical
device;
said position detector comprises a transmitter and a receiver;
said transmitter of said position detector is mounted on said
holding device; and
said receiver of said position detector is arranged on said
radiation transmitter.
6. The X-ray diagnostic apparatus as claimed in claim 1, further
comprising:
a holding device for holding the object to be examined;
wherein:
said position detector is implemented as at least any one of a
magnetic, electromagnetic, optical, acoustic or mechanical
device;
said position detector comprises a transmitter and a receiver;
said transmitter of said position detector is arranged on said
radiation transmitter; and
said receiver of said position detector is mounted on said holding
device.
7. The X-ray diagnostic apparatus as claimed in claim 1, further
comprising:
a coupling between said radiation transmitter and a coupled object,
the coupled object including at least one of said radiation
receiver and the object under examination, wherein said coupling
limits a relative displacement between said radiation transmitter
and the coupled object.
8. The X-ray diagnostic apparatus as claimed in claim 7, wherein
said coupling is an elastic coupling that exhibits a restoring
force substantially proportional to the deviation from the desired
relationship.
9. The X-ray diagnostic apparatus as claimed in claim 7, wherein
said coupling is an elastic coupling that exhibits a small
restoring torque in response to a small deviation from the desired
relationship and exhibits a large restoring torque in response to a
large deviation from the desired relationship.
10. The X-ray diagnostic apparatus as claimed in claim 1, wherein
said evaluation device (iv) evaluates whether or not the correction
signals exceed prescribed limits.
11. The X-ray diagnostic apparatus as claimed in claim 10, wherein,
if the correction signals are within the prescribed limits, said
evaluation device (v) enables said radiation transmitter to be
stimulated to emit radiation.
12. The X-ray diagnostic apparatus as claimed in claim 10, further
comprising:
an indicator connected to said evaluation device for providing an
indication if the correction signals exceed the prescribed
limits.
13. The X-ray diagnostic apparatus as claimed in claim 1, wherein
said evaluation device comprises:
a processor that evaluates, from the position signals received from
said position detector, a deviation between the positional
relationship and the desired relationship, and produces an
evaluation result;
a signal generator that generates the correction signals based on
the evaluation result.
14. The X-ray diagnostic apparatus as claimed in claim 13, wherein
said evaluation device further comprises:
a comparator that determines whether the deviation is within a
prescribed limit, and that produces a comparison result.
15. The X-ray diagnostic apparatus as claimed in claim 14, further
comprising:
an actuator controlled by the comparison result of said comparator
to stimulate said radiation transmitter to emit radiation only if
the deviation lies within the prescribed limit.
16. The X-ray diagnostic apparatus as claimed in claim 14, further
comprising:
an indicator controlled by the comparison result of said comparator
to output a signal if the deviation exceeds the prescribed
limit.
17. An X-ray diagnostic apparatus for tomosynthesis,
comprising:
a radiation transmitter and a radiation receiver provided for
generating tomosynthesis recordings of an object under examination,
said radiation transmitter and said radiation receiver being
adjustably mounted such that, when the object under examination is
transilluminated from different directions, signals are derived at
said radiation receiver for each of the directions;
an image-generating device receiving the signals in order to
calculate a tomosynthesis recording of the object under
examination;
at least one position detector provided for registering, as
position signals, a positional relationship between at least one of
(i) said radiation transmitter and said radiation receiver, (ii)
said radiation transmitter and the object under examination, and
(iii) said radiation receiver and the object under examination,
said position detector being arranged on at least one of said
radiation transmitter, said radiation receiver, and the object
under examination; and
an evaluation device provided for evaluating the position signals
from said position detector, and for generating correction signals
if the positional relationship deviates from a desired
relationship;
whereby the correction signals are supplied from said evaluation
device to said image-generating device.
Description
FIELD OF AND BACKGROUND OF THE INVENTION
The invention relates to new and useful improvements in X-ray
diagnostic devices used for generating tomosynthesis recordings,
such as tomographic images.
PCT patent publication WO 93/22 893 A1 discloses a method with
which it is possible to reconstruct an exposure of an object under
examination without knowledge of the projection angle .alpha. and
without knowledge of the geometric arrangement of radiation
transmitter, radiation receiver and focal plane. According to this
method, a reference object made of radiation-absorbing material
having a known size and known distance from the radiation receiver
is provided in the region of the radiation receiver and is
projected onto the radiation receiver during each individual
projection. The geometric arrangement and the two-dimensional
projection angle .alpha. can then be determined on the basis of the
three-dimensional imaging of the reference object onto the
radiation receiver for each individual projection. In the case
disclosed, the reference object is arranged on a bite-wing holder
which also carries the radiation receiver.
A holder for positioning a radiation transmitter of an X-ray
diagnostic apparatus for tomosynthesis is known from German patent
publication DE 44 14 689 A1. A carrying arm is coupled to the
holder. As viewed in the radiation direction, a spherical reference
object is arranged on the carrying arm in front of the object under
examination. A radiation receiver is arranged behind the object
under examination. The holder prescribes, first, the distance from
the radiation sensor to the reference object and to the radiation
receiver, and, second, the angle .alpha. of a bundle of radiation
emitted by the radiation transmitter in relation to a reference
axis of the holding device. It is further known to arrange the
radiation source adjustably in a housing to which a positioning
device for the reference object and the radiation receiver may be
coupled.
In the case of tomosynthesis, the relative positions and locations
of radiation transmitter, object under examination and radiation
receiver must be known for all of the plurality of projections
(irradiation angles). As already explained, according to WO 93/22
893 this is achieved by evaluating the image signals which are
generated when the reference object, which is rigidly connected to
the radiation receiver, is transilluminated. In this case, however,
when the object under examination is transilluminated, undesired
superimposition of the reference object onto the image signals
results. Elaborate processing is then required to calculate out the
superimposition caused by the reference object from the image
signals.
According to DE 44 14 689 A1, the information which is necessary
for reconstructing the exposure is prescribed as a result of the
construction, that is to say as a result of the fixedly prescribed
arrangement of radiation transmitter, object under examination and
radiation receiver. Because of the greater outlay in terms of
construction, the costs of such an X-ray diagnostic apparatus
increase as a result. In addition, the object under examination
must be rigidly fixed. In particular, human patients being examined
find it highly unpleasant to be rigidly restrained.
OBJECTS OF THE INVENTION
In the case of both of the solution proposals described above, it
is considered disadvantageous that changes in the positional
relationship between the object being recorded and the radiation
receiver cannot be detected or accommodated. Another drawback of
these solutions is that such changes in relative location may have
a detrimental effect on the images produced.
A first object of the invention is therefore, to provide an X-ray
diagnostic apparatus including innovations beyond those exhibited
by the apparatuses described above. It is a further object of the
invention to render X-ray diagnostic apparatuses less susceptible
to changes in position between the object to be recorded, the
radiation receiver, etc.
SUMMARY OF THE INVENTION
These and other objects are achieved by the teachings of the
independent claims. Particularly advantageous refinements of the
invention are the subject matter of the dependent claims.
An X-ray diagnostic apparatus according to the invention is capable
of generating a tomosynthesis recording and includes a radiation
transmitter that transilluminates an object under examination from
a plurality of different directions. A radiation receiver is
adjustably mounted relative to the radiation transmitter and
produces radiation signals for generating the tomosynthesis
recording of an object under examination. An image-generating
device in the apparatus receives the radiation signals for each of
the plurality of different directions and computes the
tomosynthesis recording of the object under examination. Further,
at least one position detector is included, which produces position
signals. The position signals describe an actual positional
relationship between one or more of the radiation transmitter, the
radiation receiver, and/or the object under examination. The
position detector can be mounted on the radiation transmitter, the
radiation receiver, or the object under examination or on any
combination of these. An evaluation device in the apparatus
evaluates the position signals to determine a deviation between the
actual positional relationship and a predetermined, desired
relationship. The evaluation device additionally generates
correction signals in accordance with the evaluated deviation and
outputs the correction signals to the image-generating device.
One advantage of the invention lies in the use of position
detectors to register the relative location between the radiation
transmitter, the radiation receiver and/or the object under
examination. As a result, no mechanically rigid coupling has to be
provided. Also, the need for a reference object that first has to
be superimposed and then processed out is avoided.
According to preferred embodiments, either some form of coupling,
or no coupling, can be provided between two or more of the
radiation transmitter, the radiation receiver and the object under
examination. Examples of couplings include a coupling device that
limits the amount of permissible relative shift and an elastic
coupling device. By providing such a coupling between the radiation
transmitter, the radiation receiver, and/or the object under
examination, the object under examination is permitted a certain
freedom of movement. Particularly advantageous is a coupling in
which a small restoring torque acts in the event of small
deviations from a desired reference position and a large restoring
torque acts in the event of large deviations.
According to the invention, correction signals are generated
depending on the relationship between the radiation transmitter
and/or the radiation receiver and/or the object under examination.
Another preferred embodiment provides the further advantage that
the radiation transmitter is stimulated to emit radiation only if
prescribed limits on the correction signals are not exceeded. Yet
another embodiment gives an indication, such as an audible or
visible signal, if the correction signals exceed the prescribed
limits. The invention thereby ensures that blur-free recordings are
reliably obtained. This provides yet another related advantage, in
that radiation stress resulting from repeated transilluminations of
the object under examination, resulting from multiple attempts at
obtaining satisfactory recordings, is significantly reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and further advantageous refinements of the invention
according to the features of the dependent claims are explained in
more detail below with the aid of diagrammatic, exemplary
embodiments in the drawing, in which:
FIG. 1 shows an X-ray diagnostic apparatus for tomosynthesis
according to one embodiment of the invention, in a basic
illustration,
FIG. 2 shows a block diagram of an X-ray diagnostic apparatus
according to another embodiment of the invention,
FIG. 3 shows an exemplary embodiment that detects the movement of
an object under examination in relation to the radiation
receiver,
FIG. 4 shows a position detector arranged on the object under
examination,
FIG. 5 also shows a position detector arranged on the object under
examination,
FIG. 6 shows a device having position detectors according to FIG. 5
in a plan view,
FIG. 7 shows a coupling device of the X-ray diagnostic apparatus
according to FIG. 1, and
FIG. 8 is a functional block diagram illustrating an internal
operation of a control computer of the X-ray diagnostic
apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates one embodiment of an X-ray diagnostic apparatus
for tomosynthesis. The apparatus has a radiation transmitter 1, to
which, in this embodiment, the receiver 2 of a position detector is
mounted. The radiation transmitter 1 is associated with a radiation
receiver 3, which receives the radiation originating from the
radiation transmitter 1. The transmitter 4 of the position detector
is assigned to this radiation receiver 3. In the present
embodiment, the transmitter 4 is implemented as an infrared
transmitter, whose light is converted by the receiver 2 into
electrical signals. These electrical signals are fed to an
evaluation device 11 (see FIG. 2), which uses these electrical
signals to determine the distance, the irradiation angle and the
irradiation direction. The voltage from a voltage source 5 is
supplied to the transmitter 4. In this embodiment, both the
radiation receiver 3 and the transmitter 4 are arranged on a holder
6 implemented as a bite-wing holder.
While the above embodiment implements the position detector using
an infrared transmitter and sensor, within the context of the
invention, the position detector may be implemented in a wide
variety of ways. Examples include magnetic, electromagnetic,
optical, acoustic or mechanical devices. Furthermore, the
arrangement for the position detector is also very versatile. Thus,
e.g., contrary to the embodiment of FIG. 1, the transmitter 4 of
the position detector can be assigned to the radiation transmitter
1 while the receiver 2 of the position detector is assigned to the
radiation receiver 3. According to yet another configuration, the
transmitter 4 and receiver 3 can, for example, both be provided on
the radiation transmitter 1. This is accomplished, e.g., by having
the transmitter 4 transmit a beam of radiation that is reflected by
a reference point assigned to the radiation receiver 3 and having
the reflected radiation be received by a receiver 2 that is also
provided on the radiation transmitter 1.
As shown in FIG. 1, there need be no mechanical coupling between
the radiation transmitter 1 and the radiation receiver 3. However,
it is equally possible to provide a coupling for limiting the
amount of positional change permitted. Another possibility is an
elastic coupling which allows a certain clearance for the object
under examination to move. It is particularly advantageous if the
coupling is implemented in such a way that a small restoring force
or torque is generated for small deviations from the norm, and a
large restoring torque is generated for large deviations.
As evident from FIGS. 3, 4 and 5, the position detector need not
have only a fixed relationship to the radiation transmitter 1, as
is illustrated in FIG. 1, but can also be directly and immediately
assigned to the object under examination. As shown in FIG. 3, the
object 7 under examination, for example the jaw, is assigned the
magnets 8, whose magnetic fields are received by magnetic field
sensors 9. The magnetic field sensors 9, which can, e.g., be
provided on the radiation receiver 3 and/or on the radiation
transmitter 1, convert the magnetic fields into electrical signals
which are then fed to the evaluation unit 11. An electromagnetic
device in which either the magnetic fields are generated by coils
or else are detected by means of coils can also be used, for
instance, as the position detector. Use of changing magnetic fields
allows the measurement to be decoupled from static interfering
fields, whereby the precision of the measurement is improved.
The transmitter or receiver of the position detector can also be
arranged on the object 7 under examination, for example on the
head, and in particular on the forehead, as shown in FIG. 4.
Alternatively, the position detector can be provided on a holding
device 10 for the chin. By arranging the position detector at as
great a distance as possible from the object 7 under examination,
the measurement accuracy can be increased considerably, since when
the object 7 under examination moves out of the rest position, a
relatively large leverage, that is to say a relatively large
deflection, results.
FIG. 2 shows a further preferred embodiment of an X-ray diagnostic
apparatus according to the invention. In addition to the radiation
transmitter 1, the position detector 2, 4 and the radiation
receiver 3, the diagnostic apparatus includes a control computer 11
and an image reconstruction computer 12. The control computer 11
controls a power supply device 13 for the radiation transmitter 1,
as well controlling the transmitter. 4 of the position detector and
a signal control device 14 for the radiation receiver 3. The
control computer 11 also has connections to the image
reconstruction computer 12, to a memory 15 and to an indicator 16,
and receives signals from the receiver 2 of the position detector.
Associated with the image reconstruction computer 12 are a further
memory 17, operating elements 18, an output unit 19 and an
analog/digital converter 20 which converts the signals originating
from the radiation receiver 3.
As already explained, the control computer 11 calculates the
distance of the radiation transmitter 1 from the radiation receiver
3, the irradiation angle and the irradiation direction based on the
signals supplied by the position detector 2, 4. If the alignment of
the radiation transmitter 1, the object 7 under examination and the
radiation receiver 3 deviates from a desired relationship, then the
control computer 11 forms correction signals which are fed to the
image reconstruction computer 12, either immediately or at a later
time after temporary storage in the memory 15. These correction
signals are then taken into account during the calculation of a
transillumination image of the object 7 under examination.
According to an advantageous embodiment, depending on the
correction signals, the radiation transmitter 1 can be stimulated
to emit radiation only if predetermined limits on the correction
signals are not exceeded. Either in addition or as an alternative,
an indicator 16 can be provided for indicating, for example, that
no blur-free recording can be generated because of the movement of
the object 7 under examination. The indicator 16 can be, for
example, a speaker, an LED, or a display for generating an audible
and/or a visible signal.
FIG. 8 is a functional block diagram for illustrating internal
operational features of the control computer 11 according to one
embodiment of the invention. As shown, the control computer 11
includes a processing section 30, a signal generator 31 and a
comparator 32. The processing section, inter alia, receives
position signals from the receiver 2 of the position detector.
These position signals are indicative of actual positional
relationships between the transmitter 4 and the receiver 2 of the
position detector. Based on the position signals received, the
processing section 30 calculates the amount of deviation between
the actual, momentary positional relationship and a pre-set,
desired relationship, e.g., in the manner described above. This
calculation yields an evaluation result, which is forwarded to the
signal generator 31 and to the comparator 32.
The signal generator 31 generates the correction signals described
above based on the evaluation result received from the processing
section 30. These correction signals are then output to the image
reconstruction computer 12 or to the memory 15 as needed.
The comparator 32 uses the evaluation result to compare whether the
amount of deviation between the actual, momentary positional
relationship and a pre-set, desired relationship is within a
prescribed, predetermined limit. If the evaluation result exceeds
the prescribed limit, this indicates that the deviation is
sufficiently large that, even with the use of the correction
signals, the reconstruction computer 12 will not be capable of
constructing satisfactory images. Accordingly, if the comparator 32
determines that the prescribed limit has been exceeded, the
comparator will output a signal preventing the power supply device
13 from stimulating the radiation transmitter 1 to emit radiation.
On the other hand, if the comparator 32 determines that the
deviation is within the prescribed limit, the comparator's output
will provide an enabling signal to the power supply device 13. The
output from the comparator 32 can additionally or alternatively be
used to actuate the indicator 16, e.g., in the manner described
previously.
In the case of a conventional panoramic X-ray apparatus for
generating tomograms, the object 7 under examination is normally
fixed as rigidly as possible on the X-ray apparatus by means of
bite-wing holders, frontal supports or earpieces. This is
unpleasant for the person being examined. On the other hand, also
conventionally, movements of the object 7 under examination lead to
a disturbance in the X-ray picture generated, which is also
undesirable. According to the invention, it is possible, as already
explained, to give the object 7 under examination a certain freedom
of movement. This is accomplished by registering the object's exact
position and spatial orientation along with its change in position
during the recording sequence, through measurement by means of a
position detector.
Correction signals are then formed in the control computer 11 from
the signals output by the position detector. The correction signals
are then used, before carrying out the tomographic merging process,
to assign the correct image contents to the object details from the
particular image layer that is desired to be imaged sharply, and to
track the location of the desired image layer to the patient's
movement. As a precondition, the resulting image signals, which
change continuously corresponding to the tomographic scanning
movement of the X-ray apparatus, must be registered in fine steps
and initially stored. It is also necessary to postpone preparing
the computerized merging for the tomogram that is desired for a
given object region until the complete image signals relating to
that object region are available.
The correction signals which are generated from the position
signals influence several of the image reconstruction parameters.
These parameters include the derivation intervals for the
individual images/individual image signals as well as the
displacement of the individual images/image signals parallel and
orthogonal to the tomographic scanning direction. The correction
signals also contain rotations of the individual images/image
signals as well as changes, differentiated with respect to
direction and position, of the magnification of the individual
images/image signals, and the combining of effective individual
images/individual image signals from a plurality of image
strips/image signal strips according to derivation intervals which
vary in the direction orthogonal to the tomographic scanning
direction.
FIG. 7 illustrates a basic coupling device 21 that limits or
elastically couples the amount of permissible relative adjustment
between the radiation transmitter 1 and the radiation receiver 3 or
the object 7 under examination. In an alternative embodiment, the
device 21 could be provided between the radiation receiver 3 and
the object 7 under examination. The coupling device 21 illustrated
in FIG. 7 has a first arm 22 which is connected to the radiation
transmitter 1. The radiation receiver 3 is arranged on a second arm
23 of the coupling device 21. The object under examination, which
is not shown in FIG. 7, is usually arranged in the region of the
radiation receiver 3. The first and second arms 22, 23 are mounted
such that they are pivotable about an axis of rotation 24. A
distance pick-up 25 may be provided for detecting the pivoting of
the arms 22, 23. Rotary encoders or strain gauges, for example, are
suitable as distance pick-ups 25. Assigned to the coupling device
21 are two distance limiters 26 which, in the present embodiment,
limit the adjustability of the second arm 23.
If a spring element 27, for example a leaf spring, is connected to
the arms 22, 23, then elastic adjustment of the arms 22, 23
relative to each other and about the axis of rotation 24 is
provided. The spring element 27 can advantageously be implemented
to produce a small restoring torque in the event of a small
deviation from the desired mutual positional relationship between
the radiation transmitter 1 and the radiation receiver 3. On the
other hand, a large restoring torque is produced in response to a
large deviation. The signals generated by the distance pick-up 25
are fed to the evaluation device 11, as already explained. Within
the context of the invention, the arms 22, 23 may also be further
adjustable about an axis at right angles to the axis of rotation
24, or in other spacial directions via a universal rotary
joint.
The above description of the preferred embodiments has been given
by way of example. From the disclosure given, those skilled in the
art will not only understand the present invention and its
attendant advantages, but will also find apparent various changes
and modifications to the structures disclosed. It is sought,
therefore, to cover all such changes and modifications as fall
within the spirit and scope of the invention, as defined by the
appended claims, and equivalents thereof.
* * * * *